Acute exposures to ozone have been shown in chamber and epidemiologic studies to produce reversible declines in pulmonary function of children. The cumulative effects of repeated ozone exposures on permanent changes in pulmonary function and respiratory symptoms have been observed in animal studies and suggested in human studies. Epidemiologic studies confirming chronic effects of ozone are currently lacking. A particular weakness of most epidemiologic studies of the health effects of air pollution has been misclassification of exposure. The proposed study would build on existing data on the health status of children in Simi Valley, California, a residential community with high ambient ozone levels, using personal monitors for ozone, to quantify individual chronic ozone exposure. The Harvard passive ozone badge has been independently tested and shown to be free of inhibiting interferences, is comparable (+/-12%) to continuous ozone reference monitors, and has a 6% between sample coefficient of variation. The sampler has been used in several exposure studies with a limit of detection of less than 200 ppb-hrs. Over a one-week integration time the sampler can reliably detect concentrations less than 2 ppb. Intensive personal and micro-environmental monitoring ozone exposures will be undertaken across a stratified sample of 320 children ranging from newborns to 10 years of age. The distribution of individual ozone exposures between children by age groups within this community will be evaluated. Age-and sex-specific models will be developed to estimate chronic exposure based on ambient concentrations of ozone as modified by individual, home, spatial, and seasonal characteristics. Heterogeneity of individual chronic ozone exposure will be evaluated. The error variance of the chronic ozone exposure estimates will be quantified through the use of a validation sample. These estimates of heterogeneity and error variance of exposure will be used to evaluate the feasibility of a separate follow- up study of the health effects of chronic ozone exposures among these children.